Joint return for divorced couple

rickwm Member Newcomer

A couple I know were recently divorced, but since they were still married on
12/31/2023, their settlement agreement requires them to file a joint tax
return for 2023 and to split any refund or costs 50-50.  I just did a
run-through on their taxes, and it turns out they will owe around $7000
together, which means each of them will owe $3500 according to their
commitment in the divorce agreement.  It appears that one or perhaps both of
them will have trouble coming up with these amounts by April 15th.  What
options do they have in the following scenarios?

Scenario 1:  One party can pay the $3500 but the other cannot.   Will the
IRS accept a partial payment of the $3500 from the person who can pay and
then set up a payment plan with the other party for the remaining amount?
If the second party defaults, can the IRS come after the party was able to
pay their half?  Remember, that person fulfilled their obligation under
their divorce agreement.

Scenario 2:  If neither party can pay the full amount, can they each
negotiate a separate payment plan with the IRS or do they have to work
together on this and have the same terms?

Three other questions:

1)  What are the general options available when someone can't pay their
taxes on time?  The IRS website references a 180-day payment plan and then
some kind of longer term plan, but it doesn't state what term or terms are

2)  How does IRS determine the interest and penalties that are owed under
these plans?

3)  If the parties determine that they can't pay the total by April 15th and
have to go on a payment plan, is there any value at all in getting a filing
extension to October 15th?  Since all calculations are based on the April
15th date anyway, I'm not sure what value there would be in doing an